High pressure metal halide arc discharge lamps typically comprise an arc tube, which encloses an ionizable fill material and two electrodes at opposing ends of the tube. To reduce the time it takes to start the lamp, a starter electrode may be disposed inside the arc tube near one of the main electrodes, as shown in Freese et. al., U.S. Pat. No. 3,900,761. A discharge can be initiated between the starter electrode and one of the main electrodes at a voltage that is much lower than the voltage required to ignite an arc between the two main electrodes. The ultraviolet radiation from this discharge produces photoelectrons, which enhance gas breakdown and discharge formation in the arc tube between the two main electrodes.
Zaslavsky et al., U.S. Pat. No. 4,818,915, issued Apr. 4, 1989, discloses a UV enhancer that is separate from the arc tube. The '915 patent describes a UV enhancer that typically has a borosilicate glass envelope enclosing an ionizable fill material and a single electrode. The single electrode has a getter, which can remove certain gases when the envelope heats and outgases. When energized, the UV enhancer produces ultraviolet radiation, which illuminates the path between the main electrodes within the arc tube, thus decreasing the time for generating a high intensity arc discharge.
The starter electrode approach and the separate UV enhancer each require additional parts and manufacturing steps. The extra parts and steps add to the lamp manufacturing cost.
Another form of starting aid employs a radioactive material, usually Kr85; however, this also increases the expense of the lamp and, of course, requires special handling in the storage of the material.
U.S. Pat. No. 5,323,091 to Morris discloses a UV starting aid that is incorporated into the press seal of the arc tube.
Further, to minimize sodium loss during lamp operation, the arc tube is mounted upon a floating frame, that is, a frame that carries no electrical current. The arc tube, and a shroud if the discharge lamp is a protected lamp, is supported from the frame, which itself is mounted to at least the base of the lamp envelope. (See, for example, the above-cited '915 patent).
So-called unprotected lamps, that is, lamps without a shroud for operation in an enclosed fixture, occasionally employ supports that do function as a part of the electrical circuitry of the lamp. It is with a starting aid for the latter form of lamp that this application is primarily directed.